The present invention relates to a composite roll including at least one cemented carbide ring and nodular cast iron casing. The composite roll can be available as a complete roll or as a roll ring to be attached to a driving spindle.
Composite rolls with cemented carbide for hot or cold rolling comprise one or more cemented carbide rings with a casing of cast iron attached to a (driving) spindle by various couplings and locking devices. One problem with such rolls is that during cooling from the casting temperature the casing shrinks more than the cemented carbide ring. As a result, inwardly directed forces on the cemented carbide ring are produced, giving rise to axially directed tensile stresses on the outer surface of the cemented carbide ring, which are acting perpendicularly to microcracks generated in the roll surface during rolling. Under the influence of these tensile stresses the microcracks propagate in depth, which may cause roll breakage or need for an excessive amount of dressing, limiting the total rolling capacity of the roll.
One solution to this problem is described in U.S. Pat. No. 5,044,056, according to which one or more cemented carbide rings are cast into a casing of an essentially graphitic cast iron having bainite and preferably 15-20 weight % residual austenite, which during one or more subsequent heat treatment steps totally or partly is transformed to bainite. In this way, a favorable stress state is obtained. However, this heat treatment is a costly and time consuming operation which would be desirable to eliminate. Swedish Patent Application No. 9100405-1 corresponding to U.S. Pat. Nos. 5,248,289 and 5,359,772, discloses that even a complete roll can be made in the same way with maintained good bond between the cast iron and the cemented carbide. However, heat treatment of complete rolls with a length of up to 2500 mm requires furnaces with large dimensions and long cycle heat treatments which increases the door to door time. The resulting structure with a mix of bainite and residual austenite is also very difficult to machine.
The present invention seeks to overcome disadvantages and drawbacks of the prior art composite rolls.
According to the present invention it has been found that by using an alloy giving an as-cast material with a structure of pearlite and ferrite, the desired state of stress is provided in combination with a good metallurgical bond without subsequent heat treatment of the roll. The cast iron is easily machined in an as-cast condition and has a hardness-toughness-strength which is well balanced during use of the roll.